Flow of water is more or less similar to the flow of other fluids, including fluids such as oil or blood, fine solid particles like dust or ashes, gases and even for multiphase flow (i.e., simultaneous flow of materials with different states or phases, i.e. gas, fluid or solid, or materials with different chemical properties but in the same state or phase, i.e., fluid-fluid systems such as oil droplets in water). In this patent application, the generic term “fluid” has also been used even when it is clear from the context that the fluid in question is in fact water.
Flow of fluid in rivers and at the coast can be described through speed, swirling, and pressure over a broad time-space scale. Therefore, it is not directly possible to determine changes in all physical parameters of fluid flow. Instead, attention is paid to the main parameters important from the hydrodynamic point of view, such as speed and direction flow near the bottom.
In the case of flow of fluid with a low speed, the flow is rather laminar, i.e. the substance particles of fluid have constant speed only in the direction of flow; the laminar flow of fluid can be viewed as a sliding of several thin layers of fluid on one another (see also FIG. 3). Turbulent flow is a flow of fluid where substance particles form swirls, while the mass of substance is moving in the direction of the flow (see also FIG. 4). At certain critical speed, friction starts to cause swirls and the flow becomes turbulent. Transition from a laminar to turbulent flow is characterised by the Reynolds number:Re=UL/v<Recrt˜500-2500, where                Re—Reynolds number, U—characteristic flow speed, L—length scale of flow and v—kinematic viscosity.Laminar flow almost never occurs in natural water bodies.        
Flow speed is determined mainly for determining the flow rate of a fluid. Flow is mainly turbulent and measuring is based on the principles of turbulent flow. Flow speed fluctuates around the average value; to find the correct result, measurements must be performed during a certain period, e.g. 60-100 seconds. Generally, hydrometric rotors are used for measuring, but floats or electromagnetic measuring equipment are also utilised. At the same time, determination of flow speed and the nature of flow is necessary for controlling the objects moving in water; the flow of fluid can also be used as an energy source for watercrafts.
A known method is using a sensor based on pressure sensors inspired by the lateral line of fish for estimating flow speed in a limited range (0 to 0.5 m/s), where a sensor inspired by neuromast is tilted up to 90 degrees in the flow, compared to a state without the flow. In this solution, pressure sensors can be easily soiled and damaged upon contact with particles included in the flow of water.
US20140137664 describes a device with the float anchored in the seabed, including an accelerometer for measuring flow speed caused by waves, ocean currents, relative swell kinetics, etc. The device includes a float with a suspended accelerometer, which is anchored with a cable to the bed of a water body, whereby the accelerometer measures the tilting angle of the float. This solution requires a float and fixing device to the bed of a water body, which makes installation of the solution difficult.
New sensors with a simple design are required, sensors which are suitable for determining the speed of the flow of fluid and other parameters, such as the turbulence of flow and Reynolds number.